Electromagnetic noise generated from a flexible printed wiring board, electronic components, and the like, has an influence on other electric circuits and electronic components. This may cause a malfunction or the like. Therefore, it is necessary to shield them from electromagnetic noise. For this reason, an attempt has been made to impart an electromagnetic wave shielding function to a flexible printed wiring board.
In addition, with the miniaturization and multi-functionalization of electronic equipment equipped with a flexible printed wiring board, the space allowed for the flexible printed wiring board is becoming smaller and smaller. For this reason, it is required for a flexible printed wiring board to have a reduced thickness, to have a reduced bending radius, and to ensure its wiring conductor does not become disconnected even under severe bending conditions, so as to exhibit its functions.
As a flexible printed wiring board which has an electromagnetic wave shielding function, for example, the following have been proposed.
(1) A flexible printed wiring board, wherein an undercoat layer, a shielding layer coated with a metal powder-containing conductive paste, and an overcoat layer are sequentially provided over a copper foil wiring circuit on the surface of a heat resisting plastic film, and the ground pattern of the copper foil wiring circuit and the shielding layer are electrically connected through the undercoat layer with an appropriate space therebetween (Patent Document 1).
(2) A flexible printed wiring board, wherein an electromagnetic wave shielding film, in which a metal thin film layer and a metal filler-containing conductive adhesive layer have been sequentially provided on one surface of a cover-lay film, is mounted on a substrate film, in which an insulating layer for insulating a printed circuit excluding a part of a ground circuit has been provided, so that the conductive adhesive layer is adhered to the insulating layer and the part of the ground circuit (Patent Document 2).
However, the flexible printed wiring board of (1) involves the following problems.
(i) The metal powder-containing shielding layer is fragile since it includes numerous interfaces between different materials, and thus does not have sufficient strength against repetitive bending of the flexible wiring board.
(ii) The undercoat layer is necessary to keep the insulation between the shielding layer and the copper foil wiring circuit excluding the part of the ground pattern, and hence the flexible printed wiring board has to be thick.
(iii) It is necessary to form a through hole in a part of the undercoat layer so as to electrically connect the part of the ground pattern and the shielding layer. Thus, it takes time and labor to process the through hole.
Moreover, the flexible printed wiring board of (2) involves the following problems.
(i) The metal filler-containing conductive adhesive layer is fragile since it includes numerous interfaces between different materials, and thus does not have sufficient strength against repetitive bending of the flexible wiring board.
(ii) The insulating layer is necessary to keep the insulation between the conductive adhesive layer and the printed circuit excluding the part of the ground circuit, and hence the flexible printed wiring board has to be thick.
(iii) It is necessary to form a through hole in a part of the insulating layer so as to electrically connect the part of the ground pattern and the conductive adhesive layer. Thus, it takes time and labor to process the through hole.